While the widespread use of information communication machines such as a wireless terminal increases, the frequencies used for communications are increasingly put into a wider frequency band from several hundred MHz of a mobile telephone, etc., to a several GHz band of a wireless LAN, etc. Although terminals compatible with various communication systems are used independently at present, future implementation of a small-sized wireless terminal compatible with various communication systems is demanded. An increase in the number of passive parts such as switches incorporated in a cabinet of a terminal is foreseen; miniaturization of the passive parts is demanded.
Among them, research and development of radio frequency electromechanical (RF-MEMS: Radio Frequency MEMS) switches manufactured according to MEMS (Micro Electro Mechanical Systems) technology is actively conducted. The electromechanical switch is a switch for mechanically switching a signal propagation path by moving a minute moving electrode. The electromechanical switch has the advantages that it has excellent radio frequency characteristics of an ultra-low loss, high isolation, etc. Since the electromechanical switch can be manufactured in a process with a good affinity for RF-IC, it can also be incorporated in RF-IC and is expected as a technology largely contributing to miniaturization of a wireless section.
A switch described in patent document 1 is known as an electromechanical switch in a related art. It is a mechanical switch with membrane-like or rod-like moving electrodes made twin or cantilever and brought into or out of contact with electrode, thereby switching a signal propagation path. Electrostatic force is often used as the driving force source of a membrane or a moving body.
Means for realizing higher-speed response and lower drive voltage of the electromechanical switch is disclosed by the inventor of an art described in patent document 2. FIG. 14 is a perspective view to show the configuration of a high-speed response and low-voltage driven electromechanical switch. A comb-teeth electrode is provided on both sides of a moving electrode 103. FIG. 15 (a) is a sectional view taken on line A-A′ in FIG. 14 and FIG. 15 (b) is a sectional view taken on line B-B′ in FIG. 14. To drive the moving electrode 103 downward, a voltage is applied between the moving electrode 103 and a fixed electrode for driving the moving electrode 103. To drive the moving electrode 103 upward, a voltage is applied between the moving electrode 102 and the fixed comb-teeth electrode and an electrostatic force is added in a slanting upper direction. Since an electrostatic force can be applied in driving the moving electrode upward, the spring force can be lessened as compared with the case where the electrode is driven only by the spring force, and it is made possible to speed up at the ON/OFF time.
Thus, in the MEMS device with structures coming in mechanical contact with each other, a phenomenon in which a malfunction or a failure occurs due to attraction of a contact interface (stiction) introduces a problem. The possible stiction cause is attraction force interaction between contact surfaces caused by humidity, charging of an interlayer insulating film, etc.; among them, it is known that the attraction force caused by humidity is large.
FIG. 16 is a drawing to show the structure of a meniscus; (a) is a sectional view to show the case where the electrode surface is hydrophilic and (b) is a sectional view to show the case where the electrode surface is hydrophobic. As shown in FIG. 16, if the opposed electrodes come in contact with each other, a liquid drop (water, etc.) existing in an atmosphere condenses on the contact interface due to a capillary phenomenon and forms a liquid crosslinked structure (meniscus). Interelectrode distance dcap at which the meniscus can exist is described in non-patent document 2 and can be represented as in the following expression (1) using Kelvin radius rK:
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                                                                                  d                  cap                                ⁡                                  (                  RH                  )                                            =                            ⁢                                                -                  2                                ⁢                                  R                  k                                ⁢                cos                ⁢                                                                  ⁢                θ                                                                                        =                            ⁢                              -                                                      2                    ⁢                                          γ                      la                                        ⁢                    v                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                        RT                    ⁢                                                                                  ⁢                                          ln                      ⁡                                              (                        RH                        )                                                                                                                                                                    =                            ⁢                                                -                                                                                    (                                                  1.08                          ⁢                                                                                                          ⁢                          nm                                                )                                            ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                              ln                      ⁡                                              (                        RH                        )                                                                                            ⁢                                  (                  ifWater                  )                                                                                        (        1        )            
θ is contact angle, γla is surface energy of liquid and gas interface, v is mole volume, R is gas constant, T is temperature, and RH is relative humidity. If the interelectrode distance is away from dcap, a meniscus is not formed. If the liquid is water, the value of dcap is 0.5 nm in RH 10% and 10 nm in RH 90% at the room temperature (27° C.).
As shown in FIG. 16 (a), if the electrode surface is hydrophilic, the contact angle θ of the meniscus with the solid surface becomes a value smaller than 90° and the curvature radius r of the curved structure of the meniscus surface becomes a positive value. In this case, Laplace pressure is dominant as the force acting between electrodes by the meniscus and can be represented as in the following expression (2) from non-patent document 2:
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                      f          L                =                              Δ            ⁢                                                  ⁢            p            ⁢                                                  ⁢            A                    =                                                    γ                la                            ⁢              A                        r                                              (        2        )            
Δp is the pressure difference between the liquid and the gas, and A is the contact area between the meniscus and the solid surface. From expression (2), if the solid surface is hydrophilic and the curvature radius of the meniscus is positive, the Laplace pressure becomes a positive value and an attraction force acts between electrodes. The force causes stiction for attracting the moving body of an MEMS device. The force generated by the meniscus (capillary force) includes surface tension and Laplace pressure; the value of the Laplace pressure is larger and is dominant.
On the other hand, as shown in FIG. 16 (b), if the electrode surface is hydrophobic, the contact angle θ′ of the meniscus with the solid surface becomes a value equal to or more than 90° and the curvature radius r of the curved structure of the meniscus surface becomes a negative value. In this case, from expression (2), the Laplace pressure becomes a negative value and a repulsion force acts between electrodes. If the electrode surface is made hydrophobic and a hydrophobic electrode structure with a repulsion force acting between electrodes can be realized, it is made possible to avoid occurrence of stiction in the MEMS device.
A hydrophobic surface can be provided by forming the electrode surface of a material with low surface energy. A self-assembled monolayer (SAM) is used as a low surface energy material; it can be formed according to a low-temperature and easy method of dipping, coating, etc., on the electrode surface.
FIG. 17 is a drawing to show the electrode face structure after a monolayer is formed in a related art shown in (patent document 1); (a) is a general view and (b) is a drawing to show the dotted line part in FIG. 17 (a). A monolayer 111 is formed on a fixed electrode 113 as shown in FIG. 17 (a). The monolayer has molecules arranged on the electrode surface as self-assembled and the layer thickness is the length of a single molecule. The monolayer surface has low surface energy and becomes a hydrophobic surface with the contact angle of a liquid drop 115 being 90° or more. If a moving electrode 114 comes in contact with such an electrode surface, the Laplace pressure becomes a repulsion force and acts in a direction in which the electrodes are brought away from each other.
A silane-based material such as ODS (Octadecyltri chlorosilane) is used as disclosed in (patent document 1), and the chemical structure becomes a straight chain molecular structure shown in FIG. 17 (b). In such a monolayer, —CH3 group as the surface becomes low surface energy.    Patent document 1: DE-10355038-A1    Patent document 2: JP-A-2004-253365    Non-patent document 1: J. B. Muldavin and G. M. Rebeiz, IEEE Microwave Wireless Compon. Lett., vol. 11, pp. 334-336, August 2001.    Non-patent document 2: J. N. Israelachvili, “Intermolecular and surface forces,” Academic Press Limited, 1985.